Thousands of others were stranded on rooftops, surrounded by water left by the tsunami that washed over the low-lying farmland of the hardest-hit areas, sweeping away homes, cars, railroads and businesses.

A dam broke early Saturday local time in Fukushima Prefecture, washing away scores more houses in an area where at least 1,800 homes were destroyed by the quake, Kyodo news service said, quoting a Defense Ministry official.

Japanese Prime Minister Naoto Kan appealed for calm in the wake of his nation's worst earthquake since authorities began keeping records 140 years ago. In a nationally televised address, he said an emergency response team had been deployed and international aid was en route.

President Obama announced that another U.S. aircraft carrier was en route to assist another warship already off the coast of Japan, and other U.S. naval vessels were poised to provide relief throughout the Pacific areas inundated or threatened by the tsunami that continued to roll across the ocean.

Japan has mobilized more than 8,000 troops and 300 aircraft to evacuate those stranded and move those displaced to areas where they can be sheltered and fed, Cabinet Minister Yukio Edano said.

A Justice Department investigation will allege sweeping patterns of discrimination within the Ferguson, Mo., Police Department and at the municipal jail and court, a law enforcement official familiar with the report said Tuesday.